In jet aircraft applications, fuel is traditionally measured by weight rather than by volume. This is because the volume of a given amount of fuel varies substantially with changes in fuel temperature experienced by an aircraft during flight. The weight of the fuel provides a more reliable indication than fuel volume of how long the plane engine(s) can run on the amount of fuel remaining.
Prior art aircraft fuel systems have included sensors and systems that estimate fuel density from the dielectric constant of the fuel or the fuel temperature. From the estimate of fuel density, the weight of fuel remaining is calculated. Unfortunately, the accuracy of systems that estimate fuel weight based on the dielectric constant or temperature are limited. This is because the relationship between fuel density and the dielectric constant, as well as the relationship between fuel density and temperature has limited accuracy, especially when batch to batch fuel variations are considered.
Other types of prior art fuel density measuring systems include sensors which determine density from the frequency of a vibrating member. Typical systems of this type utilize a vibrating cylinder as a resonator which is caused to vibrate at a resonant frequency by electromagnetic forces. The vibrating cylinder, when immersed in the fuel, changes its resonant frequency in accordance with the density of the fuel. From the changes in resonant frequency, the density is estimated.
When units of this type are used in aircraft, a means of compensation must be provided to account for changes in resonant frequency that occur due to head pressure of fuel above the sensor and gas pressure above the fuel. In conventional sensor designs, pressure effects act on one side of the vibrating cylinder which is immersed in the fuel. The opposite side of the cylinder is typically housed in a vacuum in a sealed enclosure which protects the electromagnetic coils used to vibrate the cylinder. Compensating for pressure effects complicates the sensing system and increases the probabilities of inaccuracy (under vibration, for example).
Thus, there exists a need for a sensor and system that more accurately measures the density of aircraft fuel under conditions normally experienced during flight by a jet aircraft, is easier to use and economical to manufacture.